Sandwich assays generally proceed by adsorbing a target analyte onto a surface coated with a capture agent. The target analyte is then detected using a detection agent that also binds to the target analyte at a different site than the capture agent. Signal from the detection agent is used to detect the target analyte. FIGS. 1-3 are schematic illustrations of steps of a conventional sandwich assay.
FIG. 1 is a schematic illustration of a sample mixing step of a sandwich assay. A substrate 105 includes a number of capture agents 110 on a surface. A fluid sample including detection agents 112 and target analyte 114 is introduced to the surface.
FIG. 2 is a schematic illustration of an incubation step of a sandwich assay. The target analyte 114 binds to the capture agent 112. The detection agent 112 also binds to the target analyte 114. In this manner, complexes including a capture agent 110, a target analyte 114, and a capture agent 112 may be formed on the substrate 105. As shown in FIG. 2, some free detection agent 112 remains in the fluid sample and is not involved in a complex. The free detection agent 112 is not representative of the presence of target analyte, because it is not bound to target analyte. That is, the unbound detection agent may generate a false positive signal for presence of the target analyte. Accordingly, the signal from the free detection agent may obscure accurate detection. Accordingly, multiple wash steps are performed to rinse away the free detection agent, leaving only complexed detection agents bound to target analyte remaining on the substrate 105.
FIG. 3 is a schematic illustration of an amplification step of a sandwich assay. The detectable signal from the detection agent 112 bound to the substrate 105 may be too low for accurate detection. For example, the complexed detection agent 112 may be spread across too large an area of the substrate 105 to generate sufficient signal for detection. Accordingly, additional labeling agents 120 may be added and may bind to the complexes to increase the amount of signal generated by the complexes.
An example of a sandwich assay is the classical ELISA technique (enzyme linked immunosorbant assay). In ELISA, the capture and detection agents include antibodies and the target analyte is typically a protein.
Rather than a flat surface as shown in FIGS. 1-3, the surfaces of beads may be used to conduct a sandwich assay, with similar sample mixing, incubation, wash, and amplification steps.
Microfluidic systems, including “lab on a chip” or “lab on a disk” systems continue to be in development. See, Lee, B. S., et. al., “A fully automated immunoassay from whole blood on a disc,” Lab Chip 9, 1548-1555 (2009) and Madou, M. et. al., “Lab on a CD,” Annu. Rev. Biomed. Engr. 8, 601-628 (2006), which articles are hereby incorporated by reference in their entirety for any purpose.